SEDIMENT INCORPORATION INTO ARC SYSTEMS: A DETRITAL ZIRCON SURVEY OF THE METASEDIMENTARY UNITS EXPOSED IN THE NORTH CASCADES ARC, WA

Sediment entrained in arc systems has significant effects on the rheology and composition of arc crust, yet the source and timing of their incorporation remains unknown. Proposed mechanisms as to how sediment becomes entrained into arcs include forearc and/or backarc thrusting, gradual burial during arc construction, and/or relamination of subducted material. Of all exhumed magmatic continental arcs, the crystalline core of the North Cascades arguably exposes the greatest amount of metasedimentary rock, and the protolith of these rocks has yet to be identified. In order to test among sediment-entrainment models, metasedimentary units exposed in the North Cascades were sampled to conduct a detrital zircon study to correlate age signatures with possible sediment sources. Ten samples of paragneiss, schist, and phyllite were collected along the WA highway 20 corridor from the Skagit Gneiss to the Ruby Creek Heterogeneous Belt. From each sample, ~120 zircons were extracted and analyzed for U-Pb geochronology using the LA-ICPMS at UC-Santa Barbara. These analyses resulted in three age groups: (1) a paragneiss, a metagraywacke, and two phyllites with detrital ages up to ca. 280 Ma, (2) a paragneiss and two schists with Precambrian ages, and (3) a paragneiss, schist, and phyllite with only late Cretaceous to Eocene metamorphic ages, with no evidence of a detrital signature. Samples that yielded ca. 280 Ma dates are located in the northeast portion of the Skagit Gneiss and Little Jack unit and align with detrital ages from backarc Methow Basin sediments. The Precambrian age group, located entirely within the Skagit Gneiss, shows a scattered range of ages from ~100 Ma to 2.0 Ga, indicating multiple sediment sources including a more mature, cratonic component. Overall, the samples that yielded detrital ages thus suggest that multiple sources of sediment were incorporated into the arc, with burial of Methow sediments to the east, possible burial of forearc sediments to the west, such as the Naniamo group, and incorporation of Proterozoic material from the North American craton. Further dating of both possible protolith material and metasedimentary units in the crystalline core will allow more detailed characterization of the detrital zircon signatures and a better understanding of the spatial variations in sediment source.